1. Field of the Invention
The present invention relates to a mobile station capable of establishing an early radio communication connection state, a radio communication establishing method therefor, and its radio communication system.
2. Description of the Related Art
Generally, a mobile communication system includes mobile stations such as portable telephones, personal digital assistants (PDAs) or pagers, base stations, base station control stations for controlling the base stations, and a network such as the Internet or a public switched telephone network (PSTN) connected to the base station control stations.
When one mobile station receives a service such as a speech service, a packet communication service or a video telephone service, first, a radio communication connection state with one of the base stations is established, and then, this mobile station is recognized by the network. Therefore, no service is given to the mobile station before such a radio communication connection state is established.
The above-mentioned establishment of a radio communication connection state is carried out:
1) when the mobile station is switched ON or a user identity module (UIM card) or an integrated circuit (IC) card according to the 3rd Generation Mobile Telephone System is inserted thereinto to thus register the location;
2) when a cell transfer processing (handover) for changing a serving base station to its neighbor base station is expected;
3) when a handover between base stations of different systems is expected; and
4) when an intermittent reception or a sleep mode reception has completed.
Note that a cell is defined by a service area of one base station. Also, a serving cell is a cell where a mobile station is located and is currently or will communicate with the base station for this cell. Further, a neighbor cell is a cell located at the periphery of the serving cell.
In a first prior art mobile communication system (see: JP-4-344723-A), when a traffic overload occurs in the cell of one base station, the base station control station thereof reports to the network a mobile station having largest difference in received electric field strength in a memory in which the differences of received electric field strengths of radio signals from the base station and its neighbor base stations are stored. As a result, such a mobile station is selected as a candidate to be handed over, and is subject to a forced handover. Thus, the degradation of speech qualities of mobile stations can be suppressed.
In a second prior art radio communication system (see: JP-8-223110-A), in the case of occurrence of a data error, when the cause of the data error is interference, the radio channel is switched to another channel. When the cause of the data error is the collision of packets, data retransmission in a packet collision resolution mode is carried out. When the received electric field strength of a desired radio signal is lower than a predetermined value, it is determined that the distance between the mobile station and the base station is too large, so that a handover is performed upon the mobile station.
In a third prior art radio communication system (see: JP-10-12683 0-A), when the propagation environment of radio signals from a base station rapidly fluctuates, a mobile station receiving the radio signals from this base station immediately searches for neighbor cells and reports the speech qualities thereof to the network, thus carrying out a soft handover.
In a fourth prior art radio communication system (see: JP-11-289289-A), a mobile station continuously measures an electric field strength of its serving cell and electric field strengths of its neighbor cells. When the electric field strength of the serving cell is larger than a predetermined value, the mobile station stops the measurement of received electric field strengths of the neighbor cells, thus decreasing the power consumption.
In a fifth prior art radio communication system (see: JP-2002-275 19-A), the location of a mobile station is detected at every predetermined time period in accordance with a latitude and longitude of a base station for a serving cell and the latitude and longitudes of its neighbor base stations. When a handover is required, the moving direction and moving speed of the mobile station are determined in accordance with the above-mentioned information of the base station for the serving cell and its neighbor base stations to find a handover candidate from the neighbor base stations, thus enhancing the speech quality and decreasing the power consumption.
In a sixth prior art radio communication system (see: 3 GPP Organization Partners, Technical Specification “3 GPP TS 25.304 V3.12.0”, pp. 1-40, December 2002, URL: http://www.3gpp.org/ftp/specs/html-info/25304.htm), a wideband code division multiple access (WCDMA) system, a radio resource connection (RRC), a cell selection from an idle mode and a cell reselection in a control mode are disclosed.
On the other hand, a mobile station establishes a radio communication connection state with a base station having the strongest electric field strength. Such an establishing process depends upon a technical specification of the radio communication system; however, this establishing process is generally carried out by exchanging special messages.
When the above-mentioned special messages are missing on the way from the mobile station to the base station or vice versa, due to the fast moving speed of the mobile station, the situation of a street the mobile station is moving on or the fluctuation of propagation environment of radio signals, message retransmission or cell reselection is carried out. Note that, after a special paging message for establishing a radio communication connection state is transmitted from the mobile station to the base station and no response message is returned from the base station to the mobile station, a message retransmission may be carried out to retrains the same paging message and a cell reselection may be carried out to change a destinated base station of the special paging message to another base station.
However, it is not effective to continue the message retransmission for the base station from which the received electric field strength is decreased. That is, the radio resource is consumed. Also, when the number of message retransmissions is increased, it will take a longer time to start a service.
Also, in the cell reselection, it will take a longer time to change from one base station for a serving cell to another. Further, since the cell reselection is often carried out after the number of message retransmission is larger than a predetermined value, it will take a further longer time to start a service.
The above-described first, second, third, fourth, fifth and sixth prior art radio communication systems have nothing to do with the delay of start of a service due to the message retransmission and the cell reselection.